Computer audio stereo-mono amp

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I have an old computer which I want to add a stereo headphone socket to, a single speaker (mono) and pass on the line-out level stereo outputs for any additional external amps etc.

The computer has actually got two line-level outputs (stereo) and a separate mono output (on its display connector). I don't know if it would be most beneficial to use the line level stereo outputs for everything, or the mono output for the single speaker. I'd also want the speaker output to break whenever headphones were inserted into the headphone socket. I'm not sure if I should have the output level work for both headphones and speaker or just the headphones (I can also control the output level from the computer's software settings).

The single speaker placed within the computer's cabinet will mostly be emitting simple beep type alerts and sounds. I suppose around 1W should suffice. Below is a block diagram describing the above followed by a schematic from the computer's audio output section. What kind of audio amp/mixer/buffer DIY circuit, schematic or ready made circuit board would I be looking for to have the above functionality?

 

[TTL]

OK, I've given it some more thought and I think I can simplify it:
No headphone output, leave the left/right line-outputs untouched and use the "AUDIO" output of the schematic (pins 7 and 8 from the LFC-347 through a resistor each) as the speaker amp's input.

Is that output's level/impedance suitable for the input of a small (1W or so) mono amp, and would I somehow need to buffer it as that line also goes to the display output connector (DIN13 connector pin 1 as shown in the schematic below)? The standard CRT monitors for this computer came with a built-in (mono) 1W speaker and volume control, and I don't want to mess this functionality up.

Also, does anyone have a recommendation for a suitable amp schematic, kit or ready made PCB for this?